1. Field of the Invention
The present invention relates to a process for producing molded articles comprising hydraulic material and used as materials for construction, civil engineering, shipbuilding and the like, and also to molded articles comprising hydraulic materials.
2. Description of the Prior Art
There is known addition of polyvinyl alcohol (hereinafter referred to as PVA) for improving mechanical properties and moldability of molded articles comprising hydraulic materials such as cement, gypsum and granulated slug. For example, Japanese Patent Application Laid-open No. 45934/1974 discloses incorporation of PVA into pulp cement for the purpose of improving the flexural strength and impact strength of pulp cement boards, and Japanese Patent Application Laid-open Nos. 77655/1986 and 209950/1986 disclose the use of PVA to increase the flexural strength, impact strength and dimensional stability and to prevent generation of cracks of slug boards or gypsum boards.
Japanese Patent Application Laid-open Nos. 50017/1974 and 239377/1985 disclose the use of PVA for cement molded articles, and Japanese Patent Application Laid-open No. 137719/1976 disclose the use of PVA for light-weight concrete, each for the purpose of improving the mechanical properties and surface appearance and preventing generation of cracks.
The reason why PVA is used for these purposes is that PVA is stable even in systems with high pH, does not hinder hydration reaction of hydraulic materials, is water-soluble and dispersible into matrices of for example cement, and improves the properties of the hydraulic compositions by bonding particles of the hydraulic materials with each other or forming tough film of PVA in the molded articles.
Japanese Patent Application Laid-open No. 13628/1972 disclosed that addition of boron compounds to aqueous slurry compositions comprising cement and PVA powder can reduce the amount of PVA used.
Japanese Patent Application Laid-open No. 184754/1984 discloses that, in aqueous slurry compositions comprising cement and PVA powder, replacement of the PVA powder of silyl group-modified PVA improves the strength of the molded articles.
In recent years, molded articles from hydraulic materials and having the same specification have been commercially produced by autoclave-curing on a large scale. Requirements for the mass-production of molded articles from hydraulic materials are: firstly, that the product articles have high mechanical strength and dimensional stability and do not generate cracks and, secondly, high productivity. There has not been established any process for producing molded articles from hydraulic materials or any molded article from hydraulic materials that satisfies all of the above requirements.